Some theme developers may have this question. The reason to add bootstrap 3 as a base theme is, it overrides most of the theme functions to add bootstrap classes to common elements(e.g. It adds form-group & form-control classes to the form elements). This helps reduce efforts of overriding theme functions.

How to create bootstrap sub-theme?

Bootstrap theme comes with bootstrap sub-theme starter kit. You can start development of your theme with the help of starter kit by copying it in the themes directory. It is good practice to keep contributed themes like Bootstrap under contrib directory(e.g. sites/all/themes/contrib/bootstrap) & your own themes under custom directory(e.g. sites/all/themes/custom/your_theme_name).

Similar to display suite bootstrap layouts, this module provides extra layouts for panels

What we learnt?

We can develop bootstrap themes quickly with the help of Bootstrap theme as a base theme & we can enhance it with some contributed modules such as views bootstrap, Display Suite Bootstrap Layouts, Panels Bootstrap Layouts.

I hope this guide will help you build responsive themes using bootstrap 3 in Drupal 7.